Transparent strain sensors in an electronic device

ABSTRACT

One or more strain sensors can be included in an electronic device. Each strain sensor includes a strain sensitive element and one or more strain signal lines connected directly to the strain sensitive element. The strain sensor(s) are used to detect a force that is applied to the electronic device, to a component in the electronic device, and/or to an input region or surface of the electronic device. A strain sensitive element is formed or processed to have a first gauge factor and the strain signal line(s) is formed or processed to have a different second gauge factor. Additionally or alternatively, a strain sensitive element is formed or processed to have a first conductance and the strain signal line(s) is formed or processed to have a different second conductance.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 62/195,030, filed on Jul. 21, 2015,and entitled “Transparent Strain Sensors in an Electronic Device,” whichis incorporated by reference as if fully disclosed herein.

FIELD

Embodiments described herein generally relate to electronic devices.More particularly, the present embodiments relate to one or moretransparent strain sensors in an electronic device.

BACKGROUND

Strain gauges or sensors are used to detect or measure strain on anobject. Typically, the electrical resistance of a strain sensor variesin proportion to the compression and tension forces it is experiencing.The gauge factor of a strain sensor represents the sensitivity of thematerial to strain. In other words, the gauge factor indicates how muchthe resistance of the strain sensor changes with strain. The higher thegauge factor, the larger the change in resistance. Higher gauge factorsallow a greater range of strain to be detected and measured.

In some situations, it is desirable for the strain sensors to be made ofa transparent material. For example, transparent strain sensors may beused when the strain sensors are located in an area where the strainsensors can be detected visually by a user (e.g., though a display).However, some materials that are used to form transparent strain sensorshave low or zero gauge factors.

SUMMARY

One or more transparent strain sensors can be included in an electronicdevice. As used herein, the term “strain sensor” refers to a strainsensitive element and the one or more strain signal lines that connectdirectly to the strain sensitive element. In one embodiment, the strainsensor(s) are used to detect a force that is applied to the electronicdevice, to a component in the electronic device, such as an inputbutton, and/or to an input region or surface of the electronic device.In one non-limiting example, a force sensing device that includes one ormore strain sensors may be incorporated into a display stack of anelectronic device. The one or more strain sensors can be positioned inan area of the display stack that is visible to a user when the user isviewing the display. As such, the one or more transparent strain sensorscan be formed with a transparent conductive material or two or moretransparent conductive materials.

In some embodiments, each transparent strain sensitive element is formedor processed to have a first gauge factor and a first conductance. Eachtransparent strain signal line is formed or processed to have adifferent second gauge factor and a different first conductance. Forexample, in one embodiment the transparent material or materials thatform a transparent strain sensitive element may have a higher gaugefactor than the transparent material(s) of the at least one transparentstrain signal line while the conductance of the transparent strainsensitive element may be less than the conductance of the transparentstrain signal line(s). Thus, the transparent strain sensitive element isconfigured to be more sensitive to strain than the transparent strainsignal line(s) and the transparent strain signal line(s) is configuredto transmit signals more effectively.

In one aspect a transparent strain sensor includes a transparent strainsensitive element and a transparent strain signal line connecteddirectly to the strain sensitive element. The transparent strainsensitive element is formed with comprised a first transparentconductive material having a first gauge factor. The transparent strainsignal line is formed with a second transparent conductive materialhaving a second gauge factor. The first gauge factor can be greater thanthe second gauge factor in one embodiment. Additionally oralternatively, the first transparent conductive material may have afirst electrical resistance and the second transparent conductivematerial a second electrical resistance with the first electricalresistance being greater than the second electrical resistance. In anon-limiting example, the transparent strain sensitive element may beformed with a transparent GZO film or a transparent AZO film and the atleast one transparent strain signal line is formed with a transparentITO film.

In another aspect, a transparent strain sensor can be formed with ahybrid transparent conductive material that includes at least one firsttransparent conductive segment that has a first gauge factor and a firstelectrical resistance and at least one second transparent conductivesegment that has a second gauge factor and a second electricalresistance. The first transparent conductive segment is connected to thesecond transparent conductive segment. The first gauge factor can begreater than the second gauge factor and the first electrical resistancegreater than the second electrical resistance.

In another aspect, a transparent strain sensor can include a transparentstrain sensitive element and at least one transparent strain signal lineconnected directly to the transparent strain sensitive element. Thetransparent strain sensitive element and transparent strain signalline(s) can be formed with the same a transparent conductive material ormaterials, but the transparent conductive material(s) in the strainsensitive element and/or in the at least one strain signal line may bedoped with one or more dopants to change the gauge factor and/or theconductance of the transparent conductive material. Thus, thetransparent strain sensitive element and the at least one transparentstrain signal line can have different gauge factors and/or electricalconductance. In some embodiments, the gauge factor of the transparentstrain sensitive element is greater than the gauge factor of the atleast one strain signal line. Additionally or alternatively, thetransparent strain sensitive element can have a first electricalresistance and the transparent strain signal line(s) a second overallelectrical resistance where the first electrical resistance is greaterthan the second electrical resistance.

In yet another aspect, a method for producing a transparent strainsensor may include providing a transparent strain sensitive element on asubstrate and providing a transparent strain signal line that isconnected directly to the transparent strain sensitive element on thesubstrate. The transparent strain sensitive element is formed with oneor more transparent conductive materials having a first gauge factor.The transparent strain signal line is formed with one or moretransparent conductive materials having a different second gauge factor.

In another aspect, a method for producing a transparent strain sensorcan include providing a transparent strain sensitive element on asubstrate, where the transparent strain sensitive element comprises oneor more transparent conductive materials, and providing a transparentstrain signal line that is connected directly to the strain sensitiveelement on the substrate. The one or more transparent conductivematerials of the transparent strain sensitive element is processed toincrease a gauge factor of the transparent strain sensitive element. Inone non-limiting example, the one or more transparent conductivematerials may be can be laser annealed to increase the crystallinity ofthe transparent strain sensitive element, which results in a highergauge factor. In some embodiments, the transparent strain signal linecan also be formed with the same or different transparent conductivematerial(s), and the transparent conductive material(s) of thetransparent strain signal line may be processed to increase aconductance of the transparent strain signal line.

In yet another aspect, a method for producing a transparent strainsensor may include providing a transparent strain sensitive element on asubstrate and providing a transparent strain signal line that isconnected directly to the strain sensitive element on the substrate. Thetransparent strain signal line is formed with one or more transparentconductive materials. The one or more transparent conductive materialsof the transparent strain signal line can be processed to increase aconductance of the transparent strain signal line. In one non-limitingexample, the one or more transparent conductive materials may be dopedwith a dopant or dopants that reduce the overall electrical resistanceof the strain signal line, which in turn increases the conductance ofthe transparent strain signal line.

In yet another aspect, an electronic device can include a cover glassand a strain sensing structure positioned below the cover glass. Thestrain sensing structure may include a substrate, a first transparentstrain sensitive element positioned on a first surface of the substrateand a second transparent strain sensitive element positioned on a secondsurface of the substrate. One or more transparent strain signal linesare connected to each transparent strain sensitive element. In someembodiments, the first and second transparent strain sensitive elementshave a gauge factor that is greater than a gauge factor of thetransparent strain signal lines. Sense circuitry is electricallyconnected to the transparent strain signal lines, and a controller isoperably connected to the sense circuitry. The controller is configuredto determine an amount of force applied to the cover glass based on thesignals received from the sense circuitry. In some embodiments, thefirst and second transparent strain sensitive elements and thetransparent strain signal lines are positioned in an area of the displaystack that is visible to a user when the user is viewing the display.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be readily understood by the following detaileddescription in conjunction with the accompanying drawings, wherein likereference numerals designate like structural elements, and in which:

FIG. 1 depicts one example of an electronic device that can include oneor more strain sensors;

FIG. 2 depicts a plan view of an example strain sensitive structure thatis suitable for use in a display stack of an electronic device;

FIG. 3 depicts a plan view of one example of an optically transparentserpentine strain sensitive element which may be used in the examplestrain sensitive structure depicted in FIG. 2;

FIG. 4 is an enlarged view of the area shown in FIG. 2;

FIG. 5 is a flowchart of a first method for producing a strain sensor;

FIG. 6 is a simplified cross-sectional view taken along line A-A in FIG.4 of a first strain sensitive structure that is suitable for use as thestrain sensitive structure shown in FIG. 2;

FIG. 7 is a simplified cross-sectional view taken along line A-A in FIG.4 of a second strain sensitive structure that is suitable for use as thestrain sensitive structure shown in FIG. 2;

FIG. 8 is a plan view of a third example of a strain sensitive elementthat is suitable for use as the strain sensitive element shown in FIGS.2 and 4;

FIG. 9 is a flowchart of a second method for producing a strain sensor;

FIG. 10 is a flowchart of a third method for producing a strain sensor;

FIG. 11 is a flowchart of a fourth method for producing a strain sensor;

FIG. 12 is a illustrative block diagram of an electronic device that caninclude one or more strain sensors;

FIG. 13 is a cross-sectional view of an example display stack thatincludes strain sensors;

FIG. 14 is a simplified cross-sectional view of the strain sensitivestructure responding to force; and

FIG. 15 is a simplified schematic diagram of sense circuitry operablyconnected to a strain sensor.

DETAILED DESCRIPTION

Reference will now be made in detail to representative embodimentsillustrated in the accompanying drawings. It should be understood thatthe following descriptions are not intended to limit the embodiments toone preferred embodiment. To the contrary, it is intended to coveralternatives, modifications, and equivalents as can be included withinthe spirit and scope of the described embodiments as defined by theappended claims.

The following disclosure relates to an electronic device that includesone or more strain sensors configured to detect strain based on anamount of force applied to the electronic device, a component in theelectronic device, and/or an input region of the electronic device. Asone example, the one or more strain sensors can be incorporated into adisplay stack of an electronic device, and at least a portion of the topsurface of the display screen may be an input region. In someembodiments, the one or more transparent strain sensors are located inan area of the display stack that is visible to a user when the user isviewing the display. As used herein, the term “strain sensor” includes astrain sensitive element and at least one strain signal line physicallyor directly connected to the strain sensitive element. Additionally,“optically transparent” and “transparent” are defined broadly to includea material that is transparent, translucent, or not visibly discernableby the human eye.

In some embodiments, each strain sensitive element is formed orprocessed to have a first gauge factor and a first conductance. Eachstrain signal line is formed or processed to have a different secondgauge factor and a different first conductance. For example, in oneembodiment the material or materials that form a strain sensitiveelement may have a higher gauge factor than the material(s) of the atleast one strain signal line while the conductance of the strainsensitive element may be less than the conductance of the strain signalline(s). Thus, the strain sensitive element is configured to be moresensitive to strain than the strain signal line(s) and the strain signalline(s) is configured to transmit signals more effectively. In anon-limiting example, the strain sensitive element may be formed with atransparent GZO film or a transparent AZO film and the at least onestrain signal line formed with a transparent ITO film.

In some embodiments, a gauge factor and/or a conductance of a strainsensitive element or a strain signal line can be based at least in parton the structure and/or the operating conditions of the electronicdevice or a component in the electronic device that includes one or morestrain sensors.

In another embodiment, a strain sensitive element and/or the one or morestrain signal lines connected to the strain sensitive element may beprocessed after the strain sensitive elements and the strain signalline(s) are formed. In a non-limiting example, the material used to formthe strain sensitive element and the strain signal line(s) can be thesame material or materials, and the material(s) in the strain sensitiveelement and/or the material(s) in the strain signal lines is processedto adjust the conductance and/or the gauge factor of the processedcomponent. In one embodiment, the strain sensitive element can be laserannealed to increase the crystallinity of the strain sensitive element,which results in a higher gauge factor. Additionally or alternatively,the one or more strain signal lines may be doped with a dopant ordopants that reduce the overall electrical resistance of the strainsignal line(s), which in turn increases the conductance of the strainsignal line(s).

And in yet another embodiment, one or more parameters of the fabricationprocess that is used to form the strain sensitive element and/or thestrain signal line(s) may be adjusted to increase the gauge factorand/or the conductance of the component. For example, in one embodimentthe flow rate of oxygen can be increased when the strain sensitiveelement is deposited onto the substrate. The higher oxygen flow rate canreduce the carrier concentration and/or mobility of the carriers in thestrain sensitive element. In another embodiment, the thickness of thematerial used to form the strain sensitive element and/or the strainsignal line(s) may be adjusted to increase the gauge factor or theconductance of the component.

Directional terminology, such as “top”, “bottom”, “front”, “back”,“leading”, “trailing”, etc., is used with reference to the orientationof the Figure(s) being described. Because components of embodimentsdescribed herein can be positioned in a number of differentorientations, the directional terminology is used for purposes ofillustration only and is in no way limiting. When used in conjunctionwith layers of a display or device, the directional terminology isintended to be construed broadly, and therefore should not beinterpreted to preclude the presence of one or more intervening layersor other intervening features or elements. Thus, a given layer that isdescribed as being formed, positioned, disposed on or over anotherlayer, or that is described as being formed, positioned, disposed belowor under another layer may be separated from the latter layer by one ormore additional layers or elements.

These and other embodiments are discussed below with reference to FIGS.1-15. However, those skilled in the art will readily appreciate that thedetailed description given herein with respect to these Figures is forexplanatory purposes only and should not be construed as limiting.

FIG. 1 shows one example of an electronic device that can include one ormore strain sensors. In the illustrated embodiment, the electronicdevice 100 is implemented as a smart telephone. Other embodiments canimplement the electronic device differently. For example, an electronicdevice can be a laptop computer, a tablet computing device, a wearablecomputing device, a smart watch, a digital music player, a display inputdevice, a remote control device, and other types of electronic devicesthat include a strain sensor or sensors.

The electronic device 100 includes an enclosure 102 surrounding adisplay 104 and one or more input/output (I/O) devices 106 (shown asbutton). The enclosure 102 can form an outer surface or partial outersurface for the internal components of the electronic device 100, andmay at least partially surround the display 104. The enclosure 102 canbe formed of one or more components operably connected together, such asa front piece and a back piece. Alternatively, the enclosure 102 can beformed of a single piece operably connected to the display 104.

The display 104 can be implemented with any suitable display, including,but not limited to, a multi-touch sensing touchscreen device that usesliquid crystal display (LCD) technology, light emitting diode (LED)technology, organic light-emitting display (OLED) technology, or organicelectro luminescence (OEL) technology.

In some embodiments, the I/O device 106 can take the form of a homebutton, which may be a mechanical button, a soft button (e.g., a buttonthat does not physically move but still accepts inputs), an icon orimage on a display, and so on. Further, in some embodiments, the buttoncan be integrated as part of a cover glass of the electronic device.Although not shown in FIG. 1, the electronic device 100 can includeother types of I/O devices, such as a microphone, a speaker, a camera,and one or more ports such as a network communication port and/or apower cord port.

Strain sensors can be included in one or more locations of theelectronic device 100. For example, in one embodiment one or morestrains sensors may be included in the I/O device 106. The strainsensor(s) can be used to measure an amount of force and/or a change inforce that is applied to the I/O device 106. In another embodiment, oneor more strain sensors can be positioned under at least a portion of theenclosure to detect a force and/or a change in force that is applied tothe enclosure. Additionally or alternatively, one or more strainssensors may be included in a display stack for the display 104. Thestrain sensor(s) can be used to measure an amount of force and/or achange in force that is applied to the display or to a portion of thedisplay. As described earlier, a strain sensor includes a strainsensitive element and at least one strain signal line physically ordirectly connected to the strain sensitive element.

In one non-limiting example, the entire top surface of a display may bean input region that is configured to receive one or more force inputsfrom a user. FIG. 2 depicts a plan view of an example strain sensitivestructure that is suitable for use in a display stack. The strainsensitive structure 200 can include a grid of independent opticallytransparent strain sensitive elements 204 that are formed in or on asubstrate 202. The strain sensitive elements 204 may be formed in or onat least a portion of one or both surfaces of the substrate 202. Thesubstrate 202 can be formed of any suitable material or materials. Inone embodiment, the substrate 202 is formed with an opticallytransparent material, such as polyethylene terephthalate (PET).

As discussed earlier, the strain sensitive elements 204 are configuredto detect strain based on an amount of force applied to an input regionof the display. The strain sensitive elements 204 may be formed with atransparent conductive material or materials such as, for example,polyethylenedioxythiophene (PEDOT), a tin doped indium oxide (ITO) film,a gallium doped zinc oxide (GZO) film, an aluminum doped zinc oxide(AZO) film, carbon nanotubes, graphene, silver nanowire, other metallicnanowires, and the like. In certain embodiments, the strain sensitiveelements 204 may be selected at least in part on temperaturecharacteristics. For example, the material selected for transparentstrain sensitive elements 204 may have a negative temperaturecoefficient of resistance such that, as temperature increases, theelectrical resistance decreases.

In this example, the transparent strain sensitive elements 204 areformed as an array of rectilinear sensing elements, although othershapes and array patterns can also be used. In many examples, eachindividual strain sensitive element 204 may have a selected shape and/orpattern. For example, in certain embodiments, a strain sensitive element204 may be deposited in a serpentine pattern, such as the pattern shownin FIG. 3. A strain sensitive element 204 can have a different patternor configuration in other embodiments.

The strain sensitive element 204 may include at least two electrodes300, 302 that are configured to be physically or directly connected toone or more strain signal lines (not shown). The strain signal line(s)can be connected to a conductive contact 206, which operably connectsthe strain sensitive element 204 to sense circuitry (not shown). Theconductive contact 206 may be a continuous contact or can be formed insegments that surround or partially surround the array of strainsensitive elements 204. In other embodiments, a strain sensitive element204 may be electrically connected to sense circuitry without the use ofelectrodes. For example, a strain sensitive element 204 may be connectedto the sense circuitry using conductive traces that are formed as partof a film layer.

Referring now to FIG. 4, there is shown an enlarged view of the area 208shown in FIG. 2. The electrodes 300, 302 of each strain sensitiveelement 204 are connected to the conductive contact 206 using strainsignal lines 400, 402, respectively. Together the strain sensitiveelement 204 and the strain signal lines 400, 402 physically or directlyconnected to the strain sensitive element 204 form a strain sensor 404.In some embodiments, a gauge factor and/or a conductance of the strainsensitive element 204 and/or the strain signal line(s) 400, 402 can bebased at least in part on the configuration of the strain sensitivestructure 200, on the operating conditions of the electronic device,and/or on the operating conditions of the component (e.g., display) inthe electronic device that includes one or more strain sensors.

FIG. 5 is a flowchart of a first method for producing a strain sensor.Initially, one or more strain sensitive elements are provided that havea material or combination of materials that have been formed orprocessed to have a first conductance and a first gauge factor (block500). Next, as shown in block 502, one or more strain signal lines thatare directly connected to each strain sensitive element are provided,where the strain signal line(s) include a material or combination ofmaterials that have been formed or processed to have a different secondconductance and a different second gauge factor. Various embodiments ofsuch strain sensitive elements and strain signal line(s) are describedin more detail in conjunction with FIGS. 6-11.

In one embodiment, the material(s) of each strain sensitive element hasa lower conductance than the conductance of the material(s) of the atleast one strain signal line. For example, the material(s) of eachstrain sensitive element may have a higher electrical resistance thanthe material(s) of the at least one strain signal line. Additionally,the first gauge factor of the strain sensitive element is higher thanthe second gauge factor of the at least one strain signal line that isconnected to the strain sensitive element. Thus, the strain sensitiveelement is more sensitive to strain than the strain signal line(s) andthe strain signal line(s) is configured to transmit signals moreeffectively. In a non-limiting example, the strain sensitive element maybe formed with a transparent GZO film or a transparent AZO film and theat least one strain signal line formed with a transparent ITO film.

Referring now to FIG. 6, there is shown a simplified cross-sectionalview taken along line A-A in FIG. 4 of a first strain sensitivestructure that is suitable for use as the strain sensitive structure 200shown in FIG. 2. The strain sensitive structure 600 includes a strainsensitive element 204 disposed on a surface of the substrate 202. Thestrain sensitive element 204 is connected to at least one strain signalline 400. As described earlier, the strain sensitive element 204 is madeof a material or combination of materials that has a first conductanceand a first gauge factor and the at least one strain signal line 400 ismade of a material or combination of materials having a different secondconductance and a different second gauge factor.

The at least one strain signal line 400 is connected to the conductivecontact 206. In some embodiments, the conductive contact is made ofcopper and is positioned in a non-visible area of an electronic device(e.g., in a non-visible area of a display). A dielectric or insulatinglayer 602 may be disposed over at least a portion of the at least onestrain signal line 400 and the strain sensitive element 204. Theinsulating layer 602 may act as a protective layer for the strain signalline 400 and the strain sensitive element 204. In embodiments where thestrain sensitive element and the strain signal line(s) are formed with asubstantially transparent material or materials, the insulating layer602 can be made of a material or combination of materials that has anindex of refraction that substantially matches the index of refractionof the strain sensitive element 204 and/or the at least one strainsignal line 400.

FIG. 7 is a simplified cross-sectional view taken along line A-A in FIG.4 of a second strain sensitive structure that is suitable for use as thestrain sensitive structure 200 shown in FIG. 2. The strain sensitivestructure 700 is similar to the strain sensitive structure 600 shown inFIG. 6, with the exception of the strain sensitive element 204. In theembodiment of FIG. 7, the strain sensitive element 204 is an alternatingmulti-layer transparent conductive structure that includes a layer ofinsulating material 702 positioned between two layers of conductivematerial 704. Each layer of conductive material 704 can be made of amaterial or combination of materials that has a first conductance and afirst gauge factor. In some embodiments, the multi-layer structure ofthe strain sensitive element 204 is configured to have an overallelectrical resistance that is lower than the strain sensitive element204 in FIG. 6 (e.g., a solid layer of conductive material) while stillproviding a higher gauge factor. In one non-limiting example, each layerof transparent conductive material can be formed with a transparent GZOfilm or a transparent AZO film.

The strain signal line 400 that is connected to the strain sensitiveelement 204 can be formed with a material or combination of materialsthat has a conductance and a gauge factor that are different from theoverall conductance and the gauge factor of the multi-layer structure ofthe strain sensitive element 204. As described earlier, the overallconductance of the strain sensitive element 204 may be less than theconductance of the strain signal line(s), while the gauge factor of thestrain sensitive element 204 can be greater than the gauge factor of thestrain signal line(s).

In the embodiments illustrated in FIGS. 6 and 7, the strain sensitiveelement has a higher gauge factor than the gauge factor of the at leastone strain signal line connected to the strain sensitive element. Thehigher gauge factor allows the strain sensitive element to be moresensitive to strain than the strain signal line(s). Additionally, insome embodiments the electrical conductance of the strain signal line(s)is higher than the conductance of the strain sensitive element. Due tothe higher conductance, the strain signal line or lines efficientlytransmit signals produced by the strain sensitive element to theconductive contact 206.

Referring now to FIG. 8, there is shown a plan view of a third exampleof a strain sensitive element that is suitable for use as the strainsensitive element 204 shown in FIGS. 2 and 4. The strain sensitiveelement 800 is a hybrid strain sensitive element that is formed with twoor more materials having different properties. In the illustratedembodiment, one segment 802 in the hybrid strain sensitive element 800is made of a first conductive material that has a first conductance andfirst gauge factor and another segment 804 is made of a secondconductive material that has a different second conductance and adifferent second gauge factor, where the first conductance is greaterthan the second conductance and the second gauge factor is greater thanthe first gauge factor.

The one or more strain signal lines that are directly connected to thehybrid strain sensitive element is formed or processed to have a gaugefactor and a conductance that is different from the overall gauge factorand overall conductance of the strain sensitive element. For example,the strain sensitive element has a greater overall gauge factor than thegauge factor of the at least one strain signal line. The higher overallgauge factor allows the strain sensitive element to be more sensitive tostrain than the strain signal line(s). Additionally, in some embodimentsthe electrical conductance of the strain signal line(s) is higher thanthe overall conductance of the strain sensitive element. Based on thehigher conductance, the strain signal line(s) can effectively transmitsignals produced by the strain sensitive element to the conductivecontact 206 (see FIGS. 2 and 4).

The segments 802, 804 can have the same dimensions or one segment (e.g.,segment 802) can have dimensions that are different from the dimensionsof the other segment (e.g., segment 804). For example, one segment canbe longer than another segment, which may result in a given gauge factorand/or conductance. In some embodiments, the given gauge factor can be agauge factor that is equal to or greater than a threshold gauge factor.The given gauge factor and/or conductance can be based at least in parton the structure and/or operating conditions of the electronic device ora component in the electronic device that includes one or more hybridstrain sensors. In one embodiment, at least two same segments (e.g., atleast two segments 802) can have different dimensions. Thus, at leastone segment 802 can have dimensions that differ from another segment 802and/or at least one segment 804 can have dimensions that differ fromanother segment 804. In another embodiment, all of the segments can havedifferent dimensions. And in some embodiments, the hybrid strainsensitive element 800 may be formed with three or more materials havingdifferent properties.

The embodiments of a strain sensor shown in FIGS. 6-8 are formed withtwo or more different materials. The materials are selected for a givengauge factor and/or an electrical conductance. Other embodiments canproduce a strain sensor by processing either the strain sensitiveelements and/or the one or more strain signal lines connected to thestrain sensitive elements after the strain sensitive elements and thestrain signal line(s) are formed. For example, in some embodiments thematerial used to form the strain sensitive elements and the strainsignal lines is the same material, and the strain sensitive elementsand/or the strain signal lines are processed to adjust the conductanceand/or the gauge factor of the processed component. The methods depictedin FIGS. 9 and 10 process the strain sensitive element and the strainsignal line(s) respectively.

FIG. 9 is a flowchart of a second method for producing a strain sensor.Initially, as shown in block 900, the strain sensitive element and thestrain signal line(s) that is connected to the strain sensitive elementare provided. Both the strain sensitive element and the strain signalline(s) are formed with one or more suitable materials. As describedearlier, in some embodiments the strain sensitive element and the one ormore strain signal lines are formed with the same material, such as, forexample, a transparent conducting oxide film. Next, as shown in block902, the strain sensitive element is processed to increase the gaugefactor of the strain sensitive element. The strain sensitive element maybe processed using any suitable technique that increases the gaugefactor of the strain sensitive element. For example, in one embodimentthe strain sensitive element is laser annealed to increase thecrystallinity of the strain sensitive element, which results in a highergauge factor.

FIG. 10 is a flowchart of a third method for producing a strain sensor.Initially, as shown in block 900, the strain sensitive element and thestrain signal line(s) that is connected to the strain sensitive elementare provided. Both the strain sensitive element and the strain signalline(s) are formed with one or more suitable materials. As describedearlier, in some embodiments the strain sensitive element and the one ormore strain signal lines are formed with the same material, such as, forexample, a transparent conducting oxide film.

Next, as shown in block 1000, the strain signal line(s) are processed toincrease the conductance of the one or more strain signal lines. Thestrain signal line(s) may be processed using any suitable technique thatincreases the conductance of the strain signal line(s). For example, inone embodiment the one or more strain signal lines are doped with adopant or dopants that reduce the overall electrical resistance of thestrain signal line(s), which in turn increases the conductance of thestrain signal line(s). For example, the one or more dopants can bediffused into the strain signal line(s) to increase the conductance ofthe strain signal line(s).

In still other embodiments, one or more parameters of the fabricationprocess that is used to form the strain sensitive elements and/or thestrain signal line(s) can be adjusted to increase the gauge factorand/or the conductance of the component. FIG. 11 is a flowchart of afourth exemplar method for producing a strain sensor. Initially, one ormore parameters of the process used to form a strain sensitive elementon a substrate is adjusted to produce a strain sensitive element thathas a higher gauge factor. For example, in one embodiment the flow rateof oxygen is increased when the strain sensitive element is depositedonto the substrate. The higher oxygen flow rate can reduce the carrierconcentration and/or mobility of the carriers in the strain sensitiveelement. In another embodiment, the thickness of the material used toform the strain sensitive element is adjusted to increase the gaugefactor and/or to reduce the electrical resistance of the strainsensitive element. For example, the material in a strain sensitiveelement can be formed as a thinner layer to result in a lowerresistivity.

Next, as shown in block 1102, one or more strain signal lines are formedon the substrate and connected to the strain sensitive element. One ormore parameters of the fabrication process used to form the strainsignal line(s) may be altered to increase the conductance of the strainsignal line(s). Additionally or alternatively, the one or more strainsignal lines can be processed after formation to increase theconductance of the strain signal line(s).

In some embodiments, a full sheet of a transparent conducting oxide filmcan be formed over and extend across the surface of a substrate (e.g.,substrate 202 in FIG. 2) and select regions or areas of the filmprocessed to produce the strain sensitive elements and/or the strainsignal lines. For example, a mask can be formed over a transparentconducting oxide film, where select areas of the mask that correspond tothe locations of the strain sensitive elements are removed. The exposedselect regions of the transparent conducting oxide film can then bedoped to produce the strain sensitive elements in the full sheet of thetransparent conducting oxide film. The dopant or dopants can be selectedto produce strain sensitive elements that have a given gauge factor, orthat have a gauge factor that is equal to or greater than a thresholdgauge factor. Similarly, select areas of a mask that correspond to thelocations of the strain signal lines can be removed, and the exposedselect regions of the transparent conducting oxide film can be doped toproduce the strain signal lines in the full sheet of the transparentconducting oxide film. The dopant or dopants can be selected to producestrain signal lines that have a given conductance, or that have aconductance that is equal to or greater than a threshold conductance.

In other embodiments, two or more sheets of a transparent conductingoxide film can be formed over the surface of a substrate (e.g.,substrate 202 in FIG. 2) and select regions or areas of the filmsprocessed to produce the strain sensitive elements and/or the strainsignal lines. For example, one mask having openings that correspond tothe locations of the strain sensitive elements can be formed over atleast a portion of a transparent conducting oxide film. The exposedregions of the transparent conducting oxide film can then be doped toproduce the strain sensitive elements in the full sheet of thetransparent conducting oxide film. A second mask can be formed over atleast a portion of a sheet of a conducting oxide film. The second maskcan have openings at locations that correspond to the locations of thestrain signal lines. The exposed select regions of the transparentconducting oxide film can be doped to produce the strain signal lines.

Referring now to FIG. 12, there is shown an illustrative block diagramof an electronic device that can include one or more strain sensors. Asdiscussed earlier, one or more strain sensors can be located on avariety of components and/or at one or more different locations in anelectronic device to detect a force applied on the component or on theelectronic device. The illustrated electronic device 1200 can includeone or more processing devices 1202, memory 1204, one or moreinput/output (I/O) devices 1206, a power source 1208, one or moresensors 1210, a network communication interface 1212, and a display1214, each of which will be discussed in more detail.

The one or more processing devices 1202 can control some or all of theoperations of the electronic device 1200. The processing device(s) 1202can communicate, either directly or indirectly, with substantially allof the components of the device. For example, one or more system buses1216 or other communication mechanisms can provide communication betweenthe processing device(s) 1202, the memory 1204, the I/O device(s) 1206,the power source 1208, the one or more sensors 1210, the networkcommunication interface 1212, and/or the display 1214. At least oneprocessing device can be configured to determine an amount of forceand/or a change in force applied to an I/O device 1206, the display,and/or the electronic device 1200 based on a signal received from one ormore strain sensors.

The processing device(s) 1202 can be implemented as any electronicdevice capable of processing, receiving, or transmitting data orinstructions. For example, the one or more processing devices 1202 canbe a microprocessor, a central processing unit (CPU), anapplication-specific integrated circuit (ASIC), a digital signalprocessor (DSP), or combinations of multiple such devices. As describedherein, the term “processing device” is meant to encompass a singleprocessor or processing unit, multiple processors, multiple processingunits, or other suitably configured computing element or elements.

The memory 1204 can store electronic data that can be used by theelectronic device 1200. For example, the memory 1204 can storeelectrical data or content such as audio files, document files, timingand control signals, operational settings and data, and image data. Thememory 1204 can be configured as any type of memory. By way of exampleonly, memory 1204 can be implemented as random access memory, read-onlymemory, Flash memory, removable memory, or other types of storageelements, in any combination.

The one or more I/O devices 1206 can transmit and/or receive data to andfrom a user or another electronic device. Example I/O device(s) 1206include, but are not limited to, a touch sensing device such as atouchscreen or track pad, one or more buttons, a microphone, a hapticdevice, a speaker, and/or a force sensing device 1218. The force sensingdevice 1218 can include one or more strain sensors. The strain sensor(s)can be configured as one of the strain sensors discussed earlier inconjunction with FIGS. 2-11.

As one example, the I/O device 106 shown in FIG. 1 may include a forcesensing device 1218. As described earlier, the force sensing device 1218can include one or more strain sensors that are configured according toone of the embodiments shown in FIGS. 2-11. An amount of force that isapplied to the I/O device 106, and/or a change in an amount of appliedforce can be determined based on the signal(s) received from the strainsensor(s).

The power source 1208 can be implemented with any device capable ofproviding energy to the electronic device 1200. For example, the powersource 1208 can be one or more batteries or rechargeable batteries, or aconnection cable that connects the electronic device to another powersource such as a wall outlet.

The electronic device 1200 may also include one or more sensors 1210positioned substantially anywhere on or in the electronic device 1200.The sensor or sensors 1210 may be configured to sense substantially anytype of characteristic, such as but not limited to, images, pressure,light, heat, touch, force, temperature, humidity, movement, relativemotion, biometric data, and so on. For example, the sensor(s) 1210 maybe an image sensor, a temperature sensor, a light or optical sensor, anaccelerometer, an environmental sensor, a gyroscope, a magnet, a healthmonitoring sensor, and so on. In some embodiments, the one or moresensors 1210 can include a force sensing device that includes one ormore strain sensors. The strain sensor(s) can be configured as one ofthe strain sensors discussed earlier in conjunction with FIGS. 2-11.

As one example, the electronic device shown in FIG. 1 may include aforce sensing device 1220 in or under at least a portion of theenclosure 102. The force sensing device 1220 can include one or morestrain sensors that may be configured as one of the strain sensorsdiscussed earlier in conjunction with FIGS. 2-11. An amount of forcethat is applied to the enclosure 102, and/or a change in an amount ofapplied force can be determined based on the signal(s) received from thestrain sensor(s).

The network communication interface 1212 can facilitate transmission ofdata to or from other electronic devices. For example, a networkcommunication interface can transmit electronic signals via a wirelessand/or wired network connection. Examples of wireless and wired networkconnections include, but are not limited to, cellular, Wi-Fi, Bluetooth,infrared, RFID, Ethernet, and NFC.

The display 1214 can provide a visual output to the user. The display1214 can be implemented with any suitable technology, including, but notlimited to, a multi-touch sensing touchscreen that uses liquid crystaldisplay (LCD) technology, light emitting diode (LED) technology, organiclight-emitting display (OLED) technology, organic electroluminescence(OEL) technology, or another type of display technology. In someembodiments, the display 1214 can function as an input device thatallows the user to interact with the electronic device 1200. Forexample, the display can include a touch sensing device 1222. The touchsensing device 1222 can allow the display to function as a touch ormulti-touch display.

Additionally or alternatively, the display 1214 may include a forcesensing device 1224. In some embodiments, the force sensing device 1224is included in a display stack of the display 1214. The force sensingdevice 1224 can include one or more strain sensors. An amount of forcethat is applied to the display 1214, or to a cover glass disposed overthe display, and/or a change in an amount of applied force can bedetermined based on the signal(s) received from the strain sensor(s).The strain sensor(s) can be configured as one of the strain sensorsdiscussed earlier in conjunction with FIGS. 2-11.

It should be noted that FIG. 12 is exemplary only. In other examples,the electronic device may include fewer or more components than thoseshown in FIG. 12. Additionally or alternatively, the electronic devicecan be included in a system and one or more components shown in FIG. 12is separate from the electronic device but in communication with theelectronic device. For example, an electronic device may be operativelyconnected to, or in communication with a separate display. As anotherexample, one or more applications or data can be stored in a memoryseparate from the electronic device. In some embodiments, the separatememory can be in a cloud-based system or in an associated electronicdevice.

As described earlier, a force sensing device that includes one or morestrain sensors can be included in a display stack of a display (e.g.,display 104 in FIG. 1). FIG. 13 depicts a cross-sectional view of anexample display stack that includes strain sensors. The display stack1300 includes a cover glass 1301 positioned over a front polarizer 1302.The cover glass 1301 can be a flexible touchable surface that is made ofany suitable material, such as, for example, glass, plastic, sapphire,or combinations thereof. The cover glass 1301 can act as an input regionfor a touch sensing device and a force sensing device by receiving touchand force inputs from a user. The user can touch and/or apply force tothe cover glass 1301 with one or more fingers or with another elementsuch as a stylus.

An adhesive layer 1304 can be disposed between the cover glass 1301 andthe front polarizer 1302. Any suitable adhesive can be used for theadhesive layer, such as, for example, an optically clear adhesive. Adisplay layer 1306 can be positioned below the front polarizer 1302. Asdescribed previously, the display layer 1306 may take a variety offorms, including a liquid crystal display (LCD), a light-emitting diode(LED) display, and an organic light-emitting diode (OLED) display. Insome embodiments, the display layer 1306 can be formed from glass orhave a glass substrate. Embodiments described herein include amulti-touch touchscreen LCD display layer.

Additionally, the display layer 406 can include one or more layers. Forexample, a display layer 406 can include a VCOM buffer layer, a LCDdisplay layer, and a conductive layer disposed over and/or under thedisplay layer. In one embodiment, the conductive layer may comprise anindium tin oxide (ITO) layer.

A rear polarizer 1308 may be positioned below the display layer 1306,and a strain sensitive structure 1310 below the rear polarizer 1308. Thestrain sensitive structure 1310 includes a substrate 1312 having a firstset of independent transparent strain sensors 1314 on a first surface1316 of the substrate 1312 and a second set of independent transparentstrain sensors 1318 on a second surface 1320 of the substrate 1312. Inthe illustrated embodiment, the first and second sets of transparentstrain sensors are located in an area of the display stack that isvisible to a user. As described earlier, a strain sensor includes astrain sensitive element and the one or more strain signal linesphysically or directly connected to the strain sensitive element. In theillustrated embodiment, the first and second surfaces 1316, 1320 areopposing top and bottom surfaces of the substrate 1312, respectively. Anadhesive layer 1322 may attach the substrate 1312 to the rear polarizer1308.

As described earlier, the strain sensors may be formed as an array ofrectilinear strain sensors. Each strain sensitive element in the firstset of independent strain sensors 1314 is aligned vertically with arespective one of the strain sensitive elements in the second set ofindependent strain sensors 1318. In many embodiments, each individualstrain sensitive element may take a selected shape. For example, incertain embodiments, the strain sensitive elements may be deposited in aserpentine pattern, similar to the serpentine pattern shown in FIG. 3.

A back light unit 1324 can be disposed below (e.g., attached to) thestrain sensitive structure 1310. The back light unit 1324 may beconfigured to support one or more portions of the substrate 1312 that donot include strain sensitive elements. For example, as shown in FIG. 13,the back light unit 1324 can support the ends of the substrate 1312.Other embodiments may configure a back light unit differently.

The strain sensors are typically connected to sense circuitry 1326through conductive connectors 1328. The sense circuitry 1326 isconfigured to detect changes in an electrical property of each of thestrain sensitive elements. In this example, the sense circuitry 1326 maybe configured to detect changes in the electrical resistance of thestrain sensitive elements, which can be correlated to a force that isapplied to the cover glass 1301. In some embodiments, the sensecircuitry 1326 may also be configured to provide information about thelocation of a touch based on the relative difference in the change ofelectrical resistance of the strain sensors 1314, 1318.

As described earlier, in some embodiments the strain sensitive elementsare formed with a transparent conducting oxide film. When a force isapplied to an input region (e.g., the cover glass 1301), the planarstrain sensitive structure 1310 is strained and the electricalresistance of the transparent conducting oxide film changes inproportion to the strain. As shown in FIG. 14, the force can cause thestrain sensitive structure 1310 to bend slightly. The bottom surface1400 of the strain sensitive structure 1310 elongates while the topsurface 1402 compresses. The strain sensitive elements measure theelongation or compression of the surface, and these measurements can becorrelated to the amount of force applied to the input region.

Two vertically aligned strain sensitive elements (e.g., 1330 and 1332)form a strain sensing device 1334. The sense circuitry 1326 may beadapted to receive signals from each strain sensing device 1334 anddetermine a difference in an electrical property of each strain sensingdevice. For example, as described above, a force may be received at thecover glass 1301, which in turn causes the strain sensitive structure1310 to bend. The sense circuitry 1326 is configured to detect changesin an electrical property (e.g., electrical resistance) of the one ormore strain sensing devices based on signals received from the strainsensing device(s) 1334, and these changes are correlated to the amountof force applied to the cover glass 1301.

In the illustrated embodiment, a gap 1336 exists between the strainsensitive structure 1310 and the back light unit 1324. Strainmeasurements intrinsically measure the force at a point on the topsurface 1316 of the substrate 1312 plus the force from the bottom atthat point on the bottom surface 1320 of the substrate 1312. When thegap 1336 is present, there are no forces on the bottom surface 1320.Thus, the forces on the top surface 1316 can be measured independentlyof the forces on the bottom surface 1320. In alternate embodiments, thestrain sensitive structure 1310 may be positioned above the displaylayer when the display stack 1300 does not include the gap 1336.

Other embodiments can configure a strain sensitive structuredifferently. For example, a strain sensitive structure can include onlyone set of strain sensitive elements on a surface of the substrate. Aprocessing device may be configured to determine an amount of force, ora change in force, applied to an input region based on signals receivedfrom the set of strain sensitive elements.

Referring now to FIG. 15, there is shown a simplified schematic diagramof sense circuitry operably connected to a strain sensing device. Thestrain sensing device 1334 (see FIG. 13) that includes two-verticallyaligned strain sensitive elements can be modeled as two resistorsR_(SENSE) configured as a voltage divider. A reference voltage divider1502 includes two reference resistors R_(REF). As one example, thestrain sensing device 1334 and the reference voltage divider 1502 may bemodeled as a Wheatstone bridge circuit, with the strain sensing device1334 forming one half bridge of the Wheatstone bridge circuit and thereference voltage divider 1502 forming the other half bridge of theWheatstone bridge circuit. Other embodiments can model the strain sensorand the reference resistors differently. For example, a strain sensitivestructure may include only one set of strain sensitive elements and aparticular strain sensitive element and a reference resistor can bemodeled as a Wheatstone half bridge circuit.

A first reference voltage (V_(REF) _(_) _(TOP)) is received at node 1504and a second reference voltage (V_(REF) _(_) _(BOT)) is received at node1506. A force signal at node 1508 of the strain sensing device 1334 anda reference signal at node 1510 of the reference voltage divider 1502are received by the sense circuitry 1512. The sense circuitry 1512 isconfigured to detect changes in an electrical property (e.g., electricalresistance) of the strain sensing device 1334 based on the differencesin the force and reference signals of the two voltage dividers. Thechanges can be correlated to the amount of force applied to a respectiveinput region of an electronic device (e.g., the cover glass 1201 in FIG.12).

Various embodiments have been described in detail with particularreference to certain features thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the disclosure. For example, the one or more strain sensitiveelements can be formed with a non-metal opaque material. Additionally oralternatively, the one or more strain sensitive elements can be formedon one layer and the strain signal line(s) on another layer such that astrain sensitive element and corresponding strain signal line(s) are notco-planar (on different planar surfaces). A via can be formed throughthe interposing layer or layers to produce an electrical contact betweenthe strain sensitive element and the strain signal lines.

Even though specific embodiments have been described herein, it shouldbe noted that the application is not limited to these embodiments. Inparticular, any features described with respect to one embodiment mayalso be used in other embodiments, where compatible. Likewise, thefeatures of the different embodiments may be exchanged, wherecompatible.

What is claimed is:
 1. A transparent strain sensor positioned in avisible area of an electronic device, the transparent strain sensorcomprising: a transparent strain sensitive element comprised of a firsttransparent conductive material having a first gauge factor; atransparent strain signal line connected directly to the transparentstrain sensitive element and comprised of a different second transparentconductive material having a second gauge factor, wherein the firstgauge factor is greater than the second gauge factor.
 2. The transparentstrain sensor of claim 1, wherein the first transparent conductivematerial has a first electrical resistance and the second transparentconductive material a second electrical resistance and the firstelectrical resistance is greater than the second electrical resistance.3. The transparent strain sensor of claim 1, wherein the firsttransparent conductive material and the second transparent conductivematerial each comprise a transparent conducting oxide film.
 4. Thetransparent strain sensor of claim 3, wherein the first transparentconductive material comprises one of a gallium doped zinc oxide film andan aluminum doped zinc oxide film.
 5. The transparent strain sensor ofclaim 3, wherein the second transparent conductive material comprises anindium tin oxide film.
 6. The transparent strain sensor of claim 3,wherein the first transparent conductive material comprises amulti-layer transparent conductive structure that includes an insulatinglayer disposed between two transparent conducting oxide films.
 7. Thetransparent strain sensor of claim 2, wherein the second transparentconductive material in the strain signal line is processed to producethe second electrical resistance.
 8. The transparent strain sensor ofclaim 7, wherein the second transparent conductive material is dopedwith one or more dopants to produce the second electrical resistance. 9.The transparent strain sensor of claim 1, wherein the first transparentconductive material in the strain sensitive element is processed toproduce the first gauge factor.
 10. The transparent strain sensor ofclaim 8, wherein the first transparent conductive material in the strainsensitive element is laser annealed to produce the first gauge factor.11. The transparent strain sensor of claim 1, wherein the visible areaof the electronic device comprises a display stack of the electronicdevice.
 12. A transparent strain sensor positioned in a visible area ofan electronic device, the transparent strain sensor comprising: atransparent strain sensitive element comprising a hybrid transparentconductive material that includes: a first transparent conductivesegment having a first gauge factor and a first electrical resistance;and a second transparent conductive segment connected to the firstconductive segment that has a second gauge factor and a secondelectrical resistance, wherein the first gauge factor is greater thanthe second gauge factor and the first electrical resistance is greaterthan the second electrical resistance.
 13. The transparent strain sensorof claim 12, further comprising at least one transparent strain signalline connected directly to the transparent strain sensitive element,wherein the at least one transparent signal line has a third gaugefactor and a third electrical resistance, wherein the third electricalresistance is less than the first and the second electrical resistances.14. The transparent strain sensor of claim 12, wherein the visible areaof the electronic device comprises a display stack of the electronicdevice.
 15. A method for producing a transparent strain sensor, themethod comprising: providing a transparent strain sensitive element on asubstrate, wherein the transparent strain sensitive element comprisesone or more first transparent conductive materials having a first gaugefactor; and providing a transparent strain signal line that is connecteddirectly to the transparent strain sensitive element on the substrate,wherein the transparent strain signal line comprises one or more secondtransparent conductive materials that are different from the one or morefirst transparent conductive materials and have a different second gaugefactor.
 16. The method of claim 15, wherein the one or more firsttransparent conductive materials in the transparent strain sensitiveelement has a first electrical resistance and the one or more secondtransparent conductive materials in the transparent strain signal linehas a second electrical resistance, the first electrical resistancebeing greater than the second electrical resistance.
 17. A method forproducing a transparent strain sensor, the method comprising: providinga transparent strain sensitive element on a substrate, wherein thetransparent strain sensitive element comprises one or more transparentconductive materials; providing a transparent strain signal line that isconnected directly to the transparent strain sensitive element on thesubstrate; and processing the one or more transparent conductivematerials in the transparent strain sensitive element to adjust aproperty of the one or more transparent conductive materials to increasea gauge factor of the transparent strain sensitive element.
 18. Themethod of claim 17, wherein the transparent strain signal line iscomprised of the same one or more transparent conductive materials. 19.The method of claim 18, further comprising processing the one or moretransparent conductive materials in the transparent strain signal lineto increase a conductance of the transparent strain signal line.
 20. Themethod of claim 17, wherein processing the one or more transparentconductive materials in the transparent strain sensitive element toincrease the gauge factor of the transparent strain sensitive elementcomprises laser annealing the one or more transparent conductivematerials in the transparent strain sensitive element to increase acrystallinity of the one or more transparent conductive materials toincrease the gauge factor of the transparent strain sensitive element.21. A method for producing a transparent strain sensor, the methodcomprising: providing a transparent strain sensitive element on asubstrate; providing a transparent strain signal line that is connecteddirectly to the transparent strain sensitive element on the substrate,wherein the transparent strain signal line comprises one or moretransparent conductive materials; and processing the one or moretransparent conductive materials in the transparent strain signal lineto adjust a property of the one or more transparent conductive materialsto increase a conductance of the transparent strain signal line.
 22. Themethod of claim 21, wherein processing the one or more transparentconductive materials in the transparent strain signal line to increasethe conductance of the transparent strain signal line comprises dopingthe one or more transparent conductive materials in the transparentstrain signal line with one or more dopants to reduce an electricalresistance of the one or more transparent conductive materials toincrease the conductance of the transparent strain signal line.
 23. Anelectronic device, comprising: a display stack for a display,comprising: a cover glass; and a strain sensing structure positionedbelow the cover glass, the strain sensing structure comprising: asubstrate; a first transparent strain sensitive element positioned on afirst surface of the substrate; a second transparent strain sensitiveelement positioned on a second surface of the substrate; and one or moretransparent strain signal lines connected directly to each transparentstrain sensitive element, wherein the first and the second transparentstrain sensitive elements have a gauge factor that is greater than agauge factor of the transparent strain signal lines, and wherein thefirst and the second transparent strain sensitive elements and thetransparent strain signal lines are positioned in an area that isvisible when viewing the display; sense circuitry electrically connectedto the transparent strain signal lines; and a controller operablyconnected to the sense circuitry and configured to determine an amountof force applied to the cover glass based on the signals received fromthe sense circuitry.
 24. The electronic device of claim 23, wherein thefirst and the second transparent strain sensitive elements are eachcomprised of one of a gallium doped zinc oxide film and an aluminumdoped zinc oxide film and the transparent strain signal lines arecomprised of a indium tin oxide film.